Spark gap



' SPARK GAP Filed July 5, 1963 ILUJUHHHIII'I i 1| Fig.3

INVEN TOR.

BY 0M+ ATTORNEYS LOUIS LUCAS JR.

United States Patent C) 3,292,049 SPARK GAP Louis Lucas, Jr., Boston, Mass, assignor to Edgerton,

Germeshausen & Grier, Inc., Boston, Mass, 21 corporation of Massachusetts Filed July 5, 1963, S01. No. 292,867 6 Claims. (Cl. 317-61) This invention relates to electric-discharge devices and more particularly to the use of such devices as electrical surge arresters.

Electrical surge arresters include over-voltage protectors, lightning arresters, crowbars and other similar devices in which an electrical discharge takes place whenever the voltage exceeds a predetermined level, thereby preventing the surges of voltage from damaging sensitive and expensive equipment, all of which devices I shall hereinafter refer to generically as crowbars. In the prior art, a wide variety of spark gaps have been used for this purpose. Gas-filled discharge tubes such as krytrons and thyratrons have also been employed for special crow-bar applications.

The two-electrode spark gap has long been the basic crowbar. It has been adequate for some applications but it has certain disadvantages which have limited its use. The gap spacing, that is, the distance between the electrodes, was so critical that the slightest change in this spacing caused a tremendous change in the breakdown voltage. The breakdown voltage may be defined as the mini-mum voltage level at which an arc discharge will take place across the electrodes. For this reason it was, indeed, difiicult to construct a two-electrode spark gap having an accurate predetermined breakdown voltage. Furthermore, even in cases where the gap had the desired breakdown voltage characteristic, operation of the gap produced changes in this characteristic caused by erosion and sputtering of electrode material and the presence of dust and foreign particles in the discharge space. Improvements were introduced to increase this distance without corresponding increases in the breakdown voltage by such means as using carbon electrodes and by employing active cathode material. These improvements produced the desired result but in so doing they also rendered the spark gaps more fragile thereby limiting the maximum voltages they could handle, thus shortening their useful lives. Three electrodes or triggered spark gaps have also been used but they had the disadvantages of being more expensive and requiring a keep-alive potential. Krytrons and thyratrons were also expensive and, in addition, the former was found quite fragile while the latter required additional power and circuitry for its heater.

A major disadvantge common to all these prior-art devices is their relatively slow arc-formation time. Arcform'ation time is the delay between the instant at which the voltage across the electrodes equals the breakdown voltage and the instant the discharge are actually forms across the two electrodes. In the case of a volt-age surge having a very fast rise time, devices with slow arc-formation times would permit a substantial portion of the voltage surge to attack the equipment before the crowbar fired. Thus the crowbar would fail to perform its basic function of protecting the equipment.

It is, therefore, an object of this invention to provide a crowbar that is not subject to the foregoing disadvantages.

Another object of this invention is to provide a simple, inexpensive electric discharge device having a very fast arc-formation time and so rugged that it is capable of handling high-power surges without damage. In summary, my invention is a two-electrode spark gap having a boundary of two dielectrics spanning the distance between the electrodes, along which boundary the are discharge takes place. At least one of the electrodes extends away from the boundary to permit the arc discharge to move away from the boundary after it forms. Preferred constructional details are hereinafter pointed out.

Other and further objects will be found in the follow ing specification and particularly in the appended claims.

My invention will be better understood from the following discussion taken in conjunction with the attached drawing of which:

FIGURE 1 is a perspective viw of a simple embodiment of my invention;

FIGURE 2 is a perspective view of a preferred embodiment of the invention;

FIGURES 3, 4 and 5 are views of other embodiments of my invention shown in perspective.

I have found that by filling the space between the electrodes of a crowbar with two different dielectrics, one a solid and the other a gas, the breakdown voltage and the arc-formation time are both greatly reduced. The are dischrage passes between the electrodes in the gaseous dielectric at the boundary of the two dielectrics and then tends to move away from the solid dielectric. The very rapid formation of the discharge arc makes my invention particularly useful where substantial-1y instantaneous response is essential. The reduction in breakdown voltage mans that the distance separating the two electrodes can be increased, thereby providing a means for accurately presetting the brakdown voltage for a crowbar. The simplicity of this crowbar and its constituent parts create a very rugged device capable of operating at very great power levels and maintaining the same characteristics after extensive use.

Referring first to FIGURE 1, electrodes 10 and 20 are disposed a distance d from each other across the surface of a refractory insulator 15, for example, ceramic. The electrodes 10 and 20 are preferably of a hard conductive metal, for example, tungsten or molybdenum, and have a planar portion bonded to the surface of refractory insulator 15. The electrodes 10 and 20 are shown parallel to each other thereby providing large electrode areas between which the discharge may take place. Electrodes 10 and 20 have a semi-cylindrical configuration with their flat surface bonded to insulator 15. They may also have a square or rectangular cross-section. In fact, any configuration may be used that provides a surface that may be bonded to theinsulator 15 and a portion that rises up from insulator 15.

Electrodes 10 and 20 are shown extending away from the discharge region in opposite directions to prevent accidental discharges between adjacent connectors. This design is used for convenience but it is not essential to the operation of our invention. Electrodes 10 and 20 may rapid-1y be connected to insulator 15 from the same side by properly insulating them to prevent arcing.

As an example of the crowbar of FIGURE 1, a pair of semi-cylindrical 40-mil diameter tungsten electrodes 10 20 were bonded to a high-alumina ceramic surface with a separation distance d between electrodes of 25 mils. The gaseous dielectric was air at atmosphere pressure. The breakdown voltage for this crowbar was about 1 kv. It passed all voltage surges below this level without firing. For voltage surges of 1.2 kv., arc-formation time was about 0.1 microsecond, and for surges of 2 kv., the time decreased to about 40 nanoseconds. This device passed all voltage surges below 1 kv. and yet fired within 40 nanoseconds for all surges in excess of 2 kv. regardless of how rapidly the voltage surges were rising. Even for slowly rising volt-age surges, the arc-formation time is still very rapid once the breakdown voltage is exceeded.

By changing the separation distance d the breakdown voltage is also changed. The breakdown voltage increases as the separation distance increases but the arcformation time remains relatively unchanged once the breakdown voltage is exceeded.

In all of these crowbars two features are worthy of note. First, these devices can handle very high power surges. In testing these crowbars I used peak currents in excess of 2,000 amperes. Secondly, the repeatability of these devices were excellent, that is, they exhibited the same characteristics after having been fired hundreds oftimes.

Although electrodes and 20 are shown parallel to each other, this is not a requirement of our invention, In fact, for reasons discussed more fully below with respect to FIGURE 2, there are advantages in bringing both eletrodes 10 and 20 onto insulator from the same side and then continuously increasing the separation distance d" so that the maximum distance between electrodes 10 and is at the ends thereof remote from their connectors and the minimum distance is :at the base of a truncated V. In such a configuration the discharge are forms across the surface of insulator 15 between electrodes 10 and 20 at their minimum separation distance whence the arc moves toward the maximum spacing end away from the connectors.

I have found that these crowbars are substantially independent of the pressure of the gaseous medium. If a gas other than air, or a vacuum, is to be used, it is, of course, necessary to dispose the device within an envelope. Any of the well-known ionizable gases, such as argon, xenon, krypton, neon, etc., or mixtures thereof may be used. A high vacuum may also be employed and I include a vacuum within the term gaseous dielectric. The use of ionizable gases or a vacuum affects the breakdown voltage but has little effect upon the arc-formation time. Each of the gaseous dielectrics must be independently investigated, but in general, the ionizable gases reduce the breakdown voltage which the vacuum increases it with respect to air.

A preferred embodiment of my invention is shown in FIGURE 2. Electrodes 10 and 20 are preferably hard, conductive metal electrodes such as tungsten, molybdenum or the like, and one end of each is bonded to an opposite side of refractory insulator 15, for example, a highalumina ceramic. Insulator 15 has a greater cross dimension than at least one of electrodes 10 and 20 to prevent unwanted discharges along the sides of insulator 15. Ceramic insulator 15 tapers to a surface 14 which is the shortest distance between electrodes 10 and 20, or at any rate is no greater than at any other points between electrodes 10 and 20. It is across this surface 14 that the discharge are forms when the potential across electrodes 10 and 20 exceed the dynamic breakdown voltage. Connections are made from the ends of electrodes 10 and 20 adjacent insulator 15, across the equipment to be protected. When a discharge arc forms between electrodes 10 and 20 across surface 14, current flow follows a path from the high voltage side of the line, through a conductor (not shown) to and along the electrode connected thereto, for example, electrode 10 through the discharge arc, to and along electrode 20, and through a conductor (not shown) to the low side of the line. The current flow along electrodes 10 and 20 creates a magnetic field about each electrode and a magnetic force perpendicular to the field. The magnetic fields created :by the current flow in electrodes 10 and 20 each produces a magnetic force which acts to move the discharge are to the right along the electrodes. This phenomenon is more completely described in copending application Ser. No. 218,596, filed Aug. 22, 1962, by K. J. Germeshausen and S. Goldberg, and assigned to the assignee hereof.

By causing the discharge are to move along the electrodes, erosion of the electrodes is greatly decreased, particularly at the points where the arc is initially formed.

The movement of the are along electrodes 10 and 20 greatly increases the life of the device because total erosion is decreased, erosion at the points of arc formation is reduced drastically and depositing of sputtered electrode material on the ceramic surface 14 is diminished thereby lessening the danger of arcing at voltages below the desired breakdown level. Arc motion takes place whether the spacing between the electrodes is uniform (parallel electrodes) or increasing from the arc-formation region across surface 14 (non-parallel electrodes). By increasing the separation distance between electrodes 10 and 20 as the arc passes to the right, extinguishment of the arc is facilitated once the voltage surge has decreased to the original breakdown voltage. If the duration of voltage surge is greater than the transit time of the are along the electrodes, it would normally blow out at the electrode ends and reform across surface 14. To prevent reforming of the are at surface 14, the end of electrode 20 is shown bent in an upright direction so that the arc maintains itself between the end of electrode 10 and the upright portion of electrode 20 as long as the voltage surge is present. In this event, the greatest electrode erosion takes place at the ends of the electrodes and not in the vicinity of surface 14 where the are is formed.

Although electrodes 10 and 20 may each be solid electrodes, I prefer to divide the right hand portion of these electrodes into a plurality of parallel segments. As an example, electrodes 10 and 20 are each shown having three such segments 11, 12, and 13, and 21, 22, and 23, respectively, but more or less segments may be used depending upon electrode size and strength. In any event, however, it is preferable, but not essential, to have an equal number of segments in the two electrodes. The purpose 'of these segments is to guide the discharge are along one such segment and insure that it follows a substantially straight path to the ends of electrodes 10 and 20.

' For this reason the segments extend back beyond the point where the arc is formed across surface 14. The greater the length of the electrode extensions from surface 14, the greater is the danger that, without the segments, the arc will veer off to the side and extinguish itself before it reaches the ends of the electrodes. By using segments, the are forms between an adjacent pair of segments, 11 and 21, 12 and 22, or 13 and 23, and follows along that pair to the ends thereof. Satisfactory operation can be obtained by making only one of the electrodes 10 or 20 segmented.

As I have explained above with respect to FIGURE 1, the dynamic breakdown voltage is a factor of the separation distance across surface 14, and the composition of the solid and gaseous dielectrics. This embodiment of my invention has provided excellent operational results, particularly in the area of high-energy voltage surges. It has been successfully tested with peak currents of 13,000 amperes in 2-millisecond pulses and 4,000 amperes in 6- milliseconds. Arc formation time for voltages in the 2,000 to 3,000 volt ranges has been found to be approximately 7 nanoseconds.

Extensive testing of these crowbars conclusively proved their advantages over the prior art. When a voltage surge reaching 10 kv. in 20 nanoseconds is applied to a crowbar of the type shown in FIGURE 2 and having a breakdown voltage of 1 kv., the arc-formation time is approximately 7 nanoseconds and, therefore, the peak voltage that the surge actually rises to is not 10 kv. but that portion of 10 kv. that is reached within those 7 nanoseconds or about 4.5 kv. This action of the crow bar is the efiective protective action provided for the equipment. This action and these resultsare independent of the identity of the gaseous dielectric or its pressure.

In order to reduce the breakdown voltage to a relatively low level without greatly decreasing the separation distance, for example, 500 volts or less, I have discovered that by applying a graphite or Aquadag (a trademark for a concentrated colloidal dispersion of pure electric-furnace graphite in water) coating to the surface 14, the static breakdown voltage is greatly lowered. In a device having a separation distance of 45 mils, the breakdown voltage was 1,300 volts without the graphite coating and 400 volts with the coating.

This graphite coating may be applied to the surface of the solid dielectric in any of the disclosed configurations. It is even useful in triggered spark gaps to reduce the usually very high trigger voltage. By applying the graphite to an insulator about the trigger electrode, the minimum trigger voltage may be reduced by a factor of 5 to 10. Similarly a very thin metallic coating applied, for example, by vacuum deposition will also provide similar benefits in the crowbar and the triggered spark gap.

The graphite coating also assists in reducing the arcformation time for low voltage surges where the maximum voltage is only slightly in excess of the breakdown voltage. In such cases there is a tendency for the full voltage to hold for a relatively long period of time before the discharge arc forms. This long arc-formation time may be considerably reduced by the application of graphite or the like to the surface of the solid dielectric. The long arc-formation time may also be decreased by reducing the pressure of the gaseous dielectric. The best results have been attained by using both of these methods.

As an example, in a crowbar having a 400-volt breakdown voltage, a fast-rising, low-voltage surge rising to slightly more than 400 volts might fail to cause the crowbar to fire or, if it did fire, it would take a long time to do so. By the addition of graphite, or the like, or by the reduction in internal pressure to about 40 torr, the crowbar fires and the arc-formation time is in the range of 0.1 to microseconds. By using both of these methods, the coating and the reduced pressure, the arc-formation time decreases even further, well into the nanosecond region.

FIGURE 3 shows another embodiment of my invention. The principal electrodes 10 and 20 are disposed on opposite sides of a solid dielectric whose surface 14 extends along the boundary line of the gaseous dielectric from electrode 10 to electrode 20. Note that only electrode extends beyond this boundary line while electrode 10 terminated thereat.

The crowbar of FIGURE 3 operates substantially the same as that of FIGURE 2 except that the discharge arc cannot move any substantial distance from the boundary line. It does, however, move away from the solid dielectric but its travel distance is limited because one end is always anchored to the end of electrode 10 while the other end moves along electrode 20 thereby elongating the arc to facilitate extinguishment thereof.

FIGURE 4 shows another embodiment of my invention. The principal electrodes 10 and 20 are shown in a coaxial arrangement with the surface 14 of insulator 15 interposed between them. Electrode connectors 10 and 20' are employed to make electrical connection separately to electrodes 10 and 20 respectively. The entire unit is encapsulated in an epoxy resin 16 with leads 10' and 20' extending outwardly therefrom. An open voltime 19 within the epoxy resin 16 contains the gaseous dielectric and the discharge arc forms across the boundary line between the solid and gaseous dielectrics. Note that at least one of the electrodes 10 or 20 may extend beyond the surface 14 to permit movement of the discharge are away from said surface as explained above with respect to FIGURE 3.

When the voltage surge appears at the device to be protected, it also appears across electrodes 10 and 20 via connectors 10 and 20' thereby initiating a discharge from electrode 10 along the boundary line between the solid and gaseous dielectrics to electrode 20 as hereinbefore described. The entire configuration shown in FIGURE 4 may be no longer than a commercial resistor and therefore has the advantages of small size and easy insertion into electric and electronic equipment to protect certain elements thereof or the entire system.

It is, of course, not necessary that electrode 10 be centrally located in insulator 15 although this configuration is preferred. If electrode 10 is not concentric within insulator 15, then the separation distance which controls the breakdown voltage is measured at the shortest distance between electrodes 10 and 20.

FIGURE 5 shows a crowbar in which the electrodes 10 and 2-0 are in the form of a truncated V with a gaseous dielectric disposed between the segments and a solid dielectric between the ends of the segments having the shorter spacing. The crowbar is connected across the equipment to be protected and in the presence of voltage surges in excess of the breakdown voltage, the are discharge forms across surface 14 at the boundary line between the solid and gaseous dielectrics and, once formed, it moves rapidly away from the boundary line as a result of the magnetic forces acting upon the are as explained above.

Although I have described my invention with great particularity and have disclosed some embodiments, my invention may take any of agreat number of configurations .and those shown are merely examples and all configurations employing the principles of my invention are deemed to fall within the spirit and scope of my invention as defined in the following claims.

I claim:

1. A spark gap comprising:

a solid dielectric having a substantially planar surface;

a gaseous dielectric disposed adjacent said surface;

a pair of spaced electrodes bonded to said solid dielectric and so disposed that a portion of said planar surface lies between said electrodes; and

a very thin coating of a material selected from the group consisting of graphite and metal coated on said portion of said planar surface.

2. A spark gap as in claim 1 in which said electrodes are spaced equidistantly from each other.

3. A spark gap as in claim 1 in which said electrodes are parallel to each other.

4. A spark gap as in claim 1 in which said electrodes form a truncated V.

5. A spark gap as in claim 1 in which the pressure of said gaseous dielectric is less than atmoshperic pressure.

6. A spark gap as in claim 1 in which said solid dielectric, gaseous dielectric, electrodes and coating are encapsulated within an epoxy resin.

References Cited by the Examiner UNITED STATES PATENTS 3,045,143 12/1962 Shickel 317-61 X FOREIGN PATENTS 131,778 6/ 1902 Germany. 182,058 1/ 1907 Germany. 183,914 4/1907 Germany.

MILTON O. HIRSHFIELD, Primary Examiner. R. V, LUPO, Assistant Examiner. 

1. A SPARK GAP COMPRISING: A SOLID DIELECTRIC HAVING A SUBSTANTIALLY PLANAR SURFACE; A GASEOUS DIELECTRIC DISPOSED ADJACENT SAID SURFACE; A PAIR OF SPACED ELECTRODES BONDED TO SAID SOLID DIELECTRIC AND SO DISPOSED THAT A PORTION OF SAID PLANAR SURFACE LIES BETWEEN SAID ELECTRODES; AND A VERY THIN COATING OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF GRAPHITE AND METAL COATED ON SAID PORTION OF SAID PLANAR SURFACE. 